The invention relates to a hydraulic dual-circuit brake system for automobiles, particularly passenger cars and, more specifically to systems of this general character having a brake pressure regulator, such as a reducer or limiter, with a hydraulically operated switching off device.
In order to ensure that the rear wheels of an automobile will not lock sooner than the front wheels, it is common practice to distribute the brake forces among the axles of the vehicle in such a manner that the braking effect is more fully utilized at the front axle than at the rear axle. This apportionment offers good directional stability during braking. However, this has the disadvantage that the maximum physically possible vehicle deceleration is not attained since, at least part of the time, a lower braking force is applied at the rear wheels than would be otherwise possible. These apportionment criteria have been reflected in legal requirements.
Due to the aforeindicated brake force distribution, in automobiles equipped with brake slip regulating means, i.e., an anti-skid braking system, for all wheels or all axles of the vehicle, brake slip regulation generally starts first at the wheels of the front axle of the automobile, before the maximum brake forces have been attained at the rear axle. Thus, in many cases, the maximum vehicle deceleration is not attained, even in automobiles provided with such brake slip regulation. It will be attained only when the operator deliberately increases the braking pressure of the brake system by actuating the brake pedal to such a degree that, even though brake slip regulation has already started on the front axle, the maximum brake force, or the brake slip regulation phase respectively, is attained at the rear axle.
In order to have the actual brake force distribution of the motor vehicle approximate the ideal brake force distribution, it is increasingly becoming customary to include in the brake pressure lines leading to the rear wheel brakes, brake pressure regulators, i.e., brake pressure reducers or brake pressure limiters, having a non-linear, kinked characteristic. Depending on the type of brake pressure regulator utilized, the brake pressure effective on the rear wheel brakes, corresponding to the predefined characteristic curve, is either limited to a fixed value or is maintained lower by a predefined value than the brake pressure effective on the front wheel brakes. In consideration of the fact that the ideal brake force distribution will be a function of the load, the brake pressure regulators in most cases are designed so that the kink point of the characteristic curve varies as a function of the load.
A hydraulic dual-circuit brake system incorporating a regulating device for brake slip regulation, i.e., an anti-skid braking system, as well as a load-dependent brake pressure regulator device for effecting the rear wheel brakes is widely known. Such systems are provided by the Audi 5000 type passenger cars.
Also well-known in passenger cars, e.g., of the Audi Quattro and Audi 4000 Quattro type, is a hydraulic dual-circuit brake system of the so-called black-and-white type, in which the front wheel brakes belong to one brake circuit and the rear wheel brakes to another. Such systems incorporate a regulating means for brake slip regulation, i.e., an anti-skid braking system, and having a fixedly set, not load-dependently controlled, brake pressure regulator device with a so-called hydraulically operated switching off device in the brake pressure line leading to the rear wheel brakes. The control inlet of the hydraulically operated switching off device of this brake pressure regulator device is acted on by pressure directed by a control line from the outlet of the master brake cylinder which supplies the front brake circuit independent of the rear brake circuit. Therefore, in case of failure of the front brake circuit due to a leak or some other defect, the reducing or limiting operation of the brake pressure regulator device is rendered ineffective and the pressure reduction or limitation in the rear brake circuit is suspended.